1. Field of the Invention
The present invention relates generally to an optical signal source for a fiber optic interferometric sensor, and specifically to a broadband optical signal source for a fiber optic gyroscope. The present invention provides an apparatus for stabilizing the scale factor of the light coming from the broadband fiber source in radiation environments.
2. Description of Related Art
Many fiber optic gyroscopes use a broadband fiber source to provide the light that is introduced into a fiber sensing coil for detecting rotation of the gyros. The typical broadband fiber source used in fiber optic gyros is a reverse pump, single-pass fiber source 100. Such a configuration is shown in FIG. 1. This configuration uses a pump light source 102, such as a pump laser diode, that emits light at a given wavelength which is directed through a wavelength division multiplexer (WDM) 104 that has two input leads and two output leads. One of the output leads 106 of the WDM 104 is physically connected to a length of erbium doped fiber (EDF) 108 via a splice 107. The EDF 108 is terminated at one end with a polished angle capillary tube 110 that keeps the light from being reflected back into the EDF 108.
The EDF 108 has a core that has been doped with one or more of the rare earth family of elements, such as erbium, which generates a light source by introducing an excitation signal into the EDF which in turn causes the fiber to emit a light characteristic of the dopant. When an erbium doped fiber is supplied with a source of energy being pumped into the fiber, such as for example a wavelength of 1480 nm generated by the pump laser diode 102, the electrons in the erbium absorb the energy and jump to a higher energy state. This energy may later be released as coherent laser light emitted in both directions of the EDF 108. When erbium is pumped with a laser at the appropriate wavelength, it emits a light in the 1525 to 1565 nanometer (nm) wavelength. The forward directed light exits the EDF 108 through the angled capillary 110 in such a way that it cannot be reflected back into the EDF 108, and this light is lost to the system.
The light emitted in the reverse direction is directed back towards the WDM 104. This light is at a different wavelength from the light introduced by the pump laser diode 102. The WDM 104 is optimized to separate the two wavelengths, where the light from the EDF 108 is at a wavelength such that it gets coupled into the fiber leg 112 that is not connected to the pump laser diode 102. This light, which is broadband in nature, is then coupled into the fiber optic gyro 114. In the fiber optic gyro 114, the light passes from a fiber optic coupler 117 used as a multiplexer (MUX) through a multifunction integrated optics chip 116, which forms and processes counter-propagating waves used in fiber optic rotation sensor systems. The counter-propagating waves are input into a fiber optic sensing coil 118. A phase shift between the counter-propagating waves develops as a result of rotating the fiber optic sensing coil 118. The light in the sensing coil 118 provides phase information which can be related to the gyro rotation rate through a term called scale factor. The scale factor is linearly related to the average wavelength of the light coming from the broadband fiber source 100. A typical spectrum for the broadband light source 100 described above is shown in FIG. 2.
Because of the broad spectral width of this light source, the scale factor becomes related to the weighted average of the spectrum, otherwise referred to as the centroid wavelength. It has been shown that when the erbium doped fiber 108 (and to a lesser degree the other fiber optic components, such as the sensing coil 118) is exposed to a source of ionizing radiation, a large shift in the centroid wavelength occurs, thus causing a large scale factor error. FIGS. 3 and 4 illustrate an example of this centroid wavelength shift occurring in a test performed on a double-pass EDF 108 having a length of 8 meters. The double-pass EDF 108 uses a mirror instead of an angle capillary tube 110 to reflect the emitted light from the far end of the erbium doped fiber 108. FIG. 3 shows the pre-exposure spectrum of the broadband fiber source and FIG. 4 shows the spectrum after the EDF has been exposed to gamma radiation with a strategic level dose.
The spectrum from a broadband light source is made up of a composite of several emission peaks. As the radiation damages the EDF 108, the different emission peaks experience different levels of attenuation. This has the effect of shifting the centroid wavelength. In FIG. 3, the emission peak which is prominent is centered at 1560 nanometers. As the radiation damages the fiber, the 1560 nanometer peak is attenuated more than the peak which is near 1534 nanometers. The 1534 nanometer peak becomes noticeable in FIG. 4. The centroid wavelength for these two cases shifts from 1558.628 nanometers in FIG. 3 to 1556.974 nanometers in FIG. 4. This corresponds to a scale factor shift of 1061 parts per million. For high accuracy fiber optic gyroscopes, this creates an unacceptable level of error. There is a need for stabilization of the scale factor of a broadband fiber source used in fiber optic gyros when exposed to ionizing radiation. Moreover, there is a need for an apparatus which provides scale factor stabilization of a broadband fiber source by minimizing the centroid wavelength shift of the spectrum of the broadband fiber source when exposed to ionizing radiation.